1. Field of the Invention
The invention refers to a method for detecting foreign bodies in liquid or solid material masses, especially in foods, as wall as to a device for implementing this method.
2. Description of the Prior Art
Numerous foods, especially milk products, e.g. yoghourts as well as baby food, are often produced on large scale in industrial manufacturing processes. Before filling the finished foods, a thorough examination for any foreign bodies possibly existing is required among other tasks. These foreign bodies can get into the foods during the production process on the one hand, or, on the other hand, they can exist in individual ingredients, e.g. fruit.
Within fish processing, light tables are used, e.g. for examining fish fillets. For obvious reasons, however, these tables are not suitable for examining the foods mentioned above and, in addition, they have the disadvantage that the actual testing is made by visual inspection and it cannot therefore be automated.
Moreover, there are known X-ray apparatuses using X-rays according to the same principle in which the visual inspection is made via a display screen. This method has one additional disadvantage, viz. the corresponding working places are subjected to increased intensity of radiation.
From DE-OS 40 13 402, a process for the detection of gas bubbles in liquid-filled pipes is known, as well as the device needed for this process, in which a transmitter and a receiver are positioned on either side of the pipe and transmit single impulses fixed in length and level to determine the presence of gas bubbles within the pipe through the change in the impulses after traversing the liquid. This well-known process is not suitable for the examination mentioned above since it can only detect a gas phase within a liquid phase. The difference in density between gas and liquid is much higher than that between foreign bodies and food (viscose mass and liquid mass).
From the magazine "Elektronik", Nr. 25/1991, a "particle detective" is known that works with high frequency ultrasound according to the (Doppler) principle. The disadvantage of this method is that it again is very work-intensive and cannot be automated.
Finally, in U.S. Pat. No. 4,607,520, a process and a device for the detection of bubbles in a flow of liquid is described. For this process, ultrasound impulses are sent through a designated segment of the liquid to be examined. During a time frame that is opened after the impulse running time, it is checked whether the impulses reached the receiver. If this is not the case, it is assumed that there is a lack of homogency (a gas bubble) within the designated segment. Again, this process is not suitable for the testing of food for any foreign body contents.
The invention was therefore based on the requirement to specify a method for detecting foreign bodies in viscous or liquid material masses, especially in foods, as well as a device for implementing this method and also enabling automated testing.